- You may be looking for the Kingdom of Acorn from Mobius Prime, or the alternate future Kingdom of Acorn (MxYL) from the MxYL universe.
The Kingdom of Acorn, in an alternate future version of Mobius, managed to flourish after Sonic the Hedgehog defeated Robo-Robotnik in a final battle, and the city of Mobotropolis was taken back. For many years the Kingdom of Acorn prospered, as Sally Acorn eventually married Sonic, and the monarchs gave birth to Prince Manik and Princess Sonia. The retired King Max lived with his family, helping to raise the royal twins. The Kingdom of Acorn was destroyed after Robo-Robotnik managed to build a new body for himself and nuked the capital of Mobotropolis, resulting in the death of the Royal Family. (StH: #22, #75, IYF)